In the early 2000s the preservative methylisothiazolinone (MI) was released as an individual preservative for industrial products and in 2005 permitted for use in cosmetic products. Up till then MI had only been used in combination with methylcholoroisthiazolinone (MCI). MCI/MI is one of the most frequent causes of preservative contact allergy and early studies showed that both MI and MCI are sensitizers. The prevalence of MI contact allergy is already around 1·5% and sources of exposure are associated with occupation, cosmetic products or household products. Use of MI in industrial products is not restricted and cases of occupational contact allergy to MI e.g. in painters are reported. The frequency of use of MI in cosmetics is low, around 1%, while up to 16·5% of household products were preserved with MI. We found 19 (1·5%) out of 1272 cosmetic products labelled with MI, primarily rinse-off products and analysed the concentration of MI by UV-HPLC-MS. The use concentration ranged between 2 and 100ppm. Repeated exposure to MI showed that many MI allergic patients reacted to 50ppm which is half the maximum permitted concentration of MI in cosmetics. The recent cases and prevalence studies on MI contact allergy could be the first sign of an epidemic of MI contact allergy. The development in prevalence of MI contact allergy should be closely monitored by including MI in the European Baseline Series at 2000ppm.